Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 3, 2012
Publication Date: September 20, 2012
Citation: Dunn, R.O. 2012. Thermal-oxidation of biodiesel by pressurized-differential scanning calorimetry: Effects of heating ramp rate. Energy and Fuels. 26:6015-6024. Interpretive Summary: Biodiesel, an alternative diesel fuel made from vegetable oils or animal fats, is very reactive to oxygen when placed in contact with air. Oxidative degradation during storage can compromise fuel quality and performance of biodiesel fuel. This work is a study on reaction kinetics of oxidation of biodiesel from soybean oil by non-isothermal (heat ramping) pressurized differential scanning calorimetry (P-DSC). Data were collected for biodiesel from four different producers and for biodiesel treated with oxidation inhibitors (antioxidants). Data from P-DSC analyses were employed to infer reaction parameters that were then utilized to calculate reaction rates and times at constant temperature. For neat (not treated by additives) biodiesel this work demonstrates P-DSC is a rapid and accurate analytical tool for assessing the relative ability to resist oxidation at various storage temperature. However, this work identified limitations for application of non-isothermal P-DSC in static mode (zero air-purge) to analysis of biodiesel treated with antioxidants. Results from this work will directly benefit biodiesel fuel producers, distributors and customers concerned by the effects of oxidation on fuel quality during storage.
Technical Abstract: Biodiesel, an alternative diesel fuel made from vegetable oils or animal fats with methanol or ethanol, is a mixture of relatively stable (saturated) and oxidatively unstable (unsaturated) long-chain fatty acid alkyl esters. During storage, oxidative degradation caused by contact with air is of utmost concern to fuel producers and consumers seeking to maintain good fuel quality. Extensive degradation can compromise fuel quality with respect to kinematic viscosity, acid value, peroxide value and other fuel properties. This work applies non-isothermal pressurized-differential scanning calorimetry (P-DSC) to investigate the oxidation reaction kinetics of biodiesel from soybean oil (SME). SME samples from four different producers were investigated. Air pressure = 2000 kPa (290 psig) was used to increase the molar concentration of oxygen available for reaction. P-DSC heating scans were conducted with varying heating ramp rate to measure oxidation onset temperature (OT) with the cell operating in static mode (closed system with zero air-purge). OT data from the heating scans were treated by Ozawa-Flynn-Wall (OFW) analysis to determine activation energy (Ea) and pre-exponential Z factors. Results compared well with literature values. Corresponding Ea and Z factors were then employed in an correlation to calculate reaction rate constant and time as functions of temperature. Results for one SME sample treated with antioxidants tocopherol and TBHQ did not follow expected trends with respect to Ea, reaction time or degree of conversion. It was concluded that conducting P-DSC analysis in static mode may not provide a sufficient quantity of oxygen within the measurement cell during the scan to finish the oxidation reaction in analysis of SME treated with antioxidants. Comparison of kinetic parameters inferred from P-DSC scans with data from isothermal OSI analyses at 50 deg C indicated moderate to very high conversion rates were necessary to achieve the OSI transition point.